The present invention relates to an ink-jet printer of the ink-on-demand type, and more particularly to a printing head for such a printer, which is driven by a reduced voltage.
Ink-jet printers of the ink-on-demand type include a piezoelectric element which is deformable upon application of a voltage so as to reduce the volume of a pressurization chamber for ejecting a jet of liquid ink from a nozzle which communicates with the pressurization chamber. Ink-jet printers have been attracting much attention since they consume a small amount of energy and can incorporate a multiplicity of nozzles. Although the structure for ejecting ink is quite simple, it has not been fully analyzed theoretically for the reasons that the ink ejection is effected under transient conditions, and it is difficult to measure the pressure and rate of flow of the ink because the printing head in the printer is small in size.
Various proposals have been made to determine the proper thickness of a vibration plate that contacts with the piezoelectric element to change the volume of the pressurization chamber. Most of the prior efforts base definition of the optimum thickness of the vibration plate only upon consideration of a vibratory system which is constituted jointly by the vibration plate and the piezoelectric element. According to Japanese Laid-Open Patent Publication No. 51-35231, for example, the neutral axes of the vibration plate and piezoelectric element should preferably lie in their median planes, and the thickness of the vibration plate is obtained from the equation:
(Et.sup.2) of the piezoelectric element=(Et.sup.2) of the vibration plate;
where E is the modulus of elasticity and t is the thickness of the plate.
A study of the above equation indicates that when the modulus of elasticity of the piezoelectric element is substantially the same as that of the vibration plate, the thickness of the piezoelectric element is substantially the same as that of the vibration plate.
Another prior attempt at analysis relies on a finite-element method to determine the thickness of a vibration plate which allows the maximum displacement of the plate with respect to a given applied drive voltage. This approach also focuses on the vibratory system only, with no consideration given to the ink flow passage to find the optimum thickness of the vibration plate with respect thereto. At any rate, conventional ink-jet printers of the ink-on-demand type have incorporated piezoelectric elements having a thickness tp ranging from about 0.3 mm to about 0.7 mm, and a vibration plate having a thickness tv which is substantially the same as the thickness tp of the piezoelectric element. The ink-jet printer head as disclosed in the Laid-Open Publication No. 51-35231 requires a relatively high drive voltage of 130 V, but other known ink-jet printers use a lower drive voltage, which, however, still equals several tens of volts or higher. Portable ink-jet printers powered by ordinary electric cells therefore have a voltage booster circuit which is of a high boosting ratio and hence of lower efficiency. This results in a failure to take full advantage of the low energy consumption offered by ink-jet printers.
What is needed is an ink-jet printing head which is driven at lower voltages in order to eliminate a voltage booster circuit, and assures safe operation and has high efficiency.